Field of the Invention
The present disclosure relates to an image forming apparatus which forms an image by an electrostatic latent image.
Description of the Related Art
In image forming apparatuses such as color laser beam printers, digital copiers, and the like, the image is temporarily formed on an intermediate transfer member. The image formed on the intermediate transfer member is then transferred to a recording medium (for example, a sheet). Then, the image is printed. As the intermediate transfer member, for example, a belt which is formed into an endless shape is used. Transferring the image formed on the intermediate transfer member to a sheet is called a secondary transfer.
FIG. 14 is a diagram for explaining a secondary transfer mechanism. In FIG. 14, a driven roller 110 and a secondary transfer roller 112 are arranged to face each other. When a driving roller 108 rotates at a constant speed, the intermediate transfer belt 107 moves at a constant speed. A sheet S is conveyed along with a guide 131 by a conveyance roller 130 and the secondary roller 112, which rotate at a constant speed. Then, the secondary transfer is performed. In this case, the intermediate transfer belt 107 is designed to move at the same speed at which the sheet S is conveyed, which, however, is not always the case. In the following, a particular example is explained using FIG. 15.
FIG. 15A is a diagram for explaining an arrangement of the driven roller 110 and the secondary transfer roller 112 in a case where the intermediate transfer belt 107 moves at the same speed at which the sheet S is conveyed. In this example, a longitudinal direction of the column-shaped driven roller 110 is defined as a main scanning direction. A direction which is vertical to the main scanning direction is defined as a sub-scanning direction. The sub-scanning direction corresponds to a direction in which the sheet S is conveyed. In the arrangement shown in FIG. 15A, a rotary shaft 110z of the driven roller 110 is in parallel with a rotary shaft 112z of the secondary transfer roller 112. Therefore, pressure (nip pressure) of a portion where the driving roller 110 and the secondary transfer roller 112 contact (nip portion) is constant regardless of a position in the main scanning direction (main scanning position). Thereby, the sheet S is conveyed at the same speed at which the intermediate transfer belt 107 moves.
FIG. 15B is a diagram for explaining an arrangement of the driven roller 110 and the secondary transfer roller 112 in a case where the intermediate transfer belt 107 does not move at the same speed at which the sheet S is conveyed. In the arrangement shown in FIG. 15A, a rotary shaft 110z of the driven roller 110 is not in parallel with a rotary shaft 112z of the secondary transfer roller 112. Due to this, the nip pressure between the driven roller 110 and the secondary transfer roller 112 varies depending on the main scanning position. For example, in FIG. 15B, a main scanning position (x) is shown. The distance between the shafts of the driven roller 110 and the secondary transfer roller 112 at this position is longer than that in the arrangement shown in FIG. 15. Therefore, the nip pressure at the main scanning position (x) is relatively low. Further, in FIG. 15B, a main scanning position (y) is shown. The distance between the shafts of the driven roller 110 and the secondary transfer roller 112 at this position is shorter than that in the arrangement shown in FIG. 15. Therefore, the nip pressure at the main scanning position (y) is relatively high. As mentioned, if the nip pressure varies depending on the main scanning position, the pressure applied to the sheet S by the driven roller 110 and the secondary transfer roller 112 also differs according to the main scanning direction. Further, the higher the nip pressure is, the faster the sheet S is conveyed. Also, the lower the nip pressure is, the slower the sheet S is conveyed. This is because as the nip pressure is high, the frictional force to the sheet S also enhances, which enables easy transmission of the rotational force to the sheet S. Due to this, the conveyance speed of the sheet S varies depending on the main scanning position in the arrangement shown in FIG. 15B.
FIG. 15B shows a case where a print operation is performed in a state where the nip pressure differs at every main scanning position. In this case, as the sheet S passes through the nip portion, it is rotated with its locus being a sector of a circle. As a result, a deviation is caused in the secondary transfer. For example, when printing an image shown in FIG. 16A, as shown in FIG. 16B, the shape is printed in a distorted shape. Comparing the distortion to a sector of a circle, a position at low nip pressure at the main scanning position is an outer peripheral side of the sector. Also, a position at high nip pressure at the main scanning position is an inner peripheral side of the sector. In the following, such distortion is called a sector deformation. FIG. 16B shows a sheet having experienced the sector deformation, in which distortion amount b1 and distortion amount b2 at the main scanning positions respectively are about 0.1 [mm]. Further, the distortion amount b3 at a position of a sub-scanning direction (sub-scanning position) is about 0.5 [mm]. As above, the distortion amount b3 at the sub-scanning position is several times larger than the distortion amounts b1 and b2 at the main scanning positions. Due to this, the distortion of the sub-scanning direction caused by the sector deformation largely affects print image quality, which is a problem.
To this problem, an image forming apparatus as disclosed in US Patent Application Publication No. US2007/0139715 (A1) intends to realize correction processing to the sector deformation by image data conversion processing. In particular, by detecting an output image formed on a printed sheet, deformation parameter of the sector deformation is obtained. Based on the result as derived, in the following printing operation, image data is converted in advance to cancel the occurrence of the distortion caused by the sector deformation. This is to avoid the occurrence of any defective image caused by the sector deformation.
On the other hand, the image forming apparatus as disclosed in US Patent Application Publication No. US2007/0139715 (A1) leaves a problem. In particular, according to the degree of sector deformation, there may be a case where a sector correction parameter value (correction value) for cancelling the occurrence of distortion caused by the sector (or fan-shape) deformation exceeds the estimated value. In this case, the defective image occurs. In the following, description is given when the defective image occurs.
When performing a sector correction by converting the image data, memory capacity required for the correction processing is estimated and prepared in advance. Then, frame processing or band processing, which will be described later, is performed. First, description is given with regard to the occurrence of the defective image in an image forming apparatus which performs the frame processing. The image forming apparatus which performs the frame processing comprises a frame buffer which is capable of storing image data of one page used for image forming. A size of the frame buffer, however, is finite. Therefore, in the sector correction, if a magnification ratio in the sub-scanning direction is larger than that of the estimated magnification ratio, the image data after the sector correction may not be stored in the frame buffer. In this case, the defective image occurs. In the following, description is given with regard to a particular example using FIG. 17.
FIG. 17A is a diagram which schematically represents image data (original image data) which is input in the image forming apparatus. Viewed from front of FIG. 17A, a left side of the original image data in the sub-scanning direction is defined as a side a. Similarly, a right side of the original image data in the sub-scanning direction is defined as a side b. FIG. 17B is a diagram which schematically represents a frame buffer in which image data after the sector correction is stored. Compared with the original image data in FIG. 17A, the size of the frame buffer in the main scanning direction remains the same, while the size of the frame buffer in the sub-scanning direction is 2[%] larger. FIG. 17C is a diagram which schematically represents image data after the sector correction to the original image data. In FIG. 17C, the length of the side a in FIG. 17A is reduced by 3 [%], which is represented as a side a′. Also, the length of the side b is enlarged by 3[%], which is represented as a side b′.
FIG. 17D is a diagram which schematically represents a state where a part of the image data after the sector correction as shown in FIG. 17C is too large to be stored in the frame buffer. The size of the frame buffer in the sub-scanning direction is 2[%] larger than that of the original image data in the sub-scanning direction. However, the side b is enlarged by 3[%], which is represented by “side b”. Therefore, an excess portion c, which is a part of the image data after the sector correction, is not stored in the frame buffer integrally with other parts of the image data after the sector correction. FIG. 17E is a diagram illustrating an enlarged excess portion c shown in FIG. 17D. The excess portion c is overwritten, for example, in an area that is irrelevant to the image data after the sector correction of the frame buffer. As a result, defective image occurs during printing.
Next, description is given with regard to the occurrence of the defective image in an image forming apparatus which performs the band processing. The image forming apparatus which performs the band processing comprises a small amount of band memory comprised of a few lines for image processing. In the band processing, a part of pixel lines of the image data comprising one page is stored in the band memory to effect the image processing. Therefore, in the band processing, it is not possible to perform processing which refers the image data not stored in the band memory. The number of lines referred in the sector correction depends on a difference between a max value and a minimum value of the length of the sides of the sector in the sub-scanning direction. For example, if the difference is large, the number of the lines to be referred is also increased. That is, if the number of lines to be referred during the sector correction processing process exceeds the number of lines which can be stored in the band memory, the defective image occurs during printing. In the following, a particular example is described using FIGS. 18 and 19.
FIG. 18 is a diagram showing image data (original image data) before the sector correction. The original image data shown in FIG. 18 is arranged with 12 pixels in the main scanning length and 10 pixels in the sub-scanning length. As mentioned, the image data is a set of pixels arranged in the main scanning direction and the sub-scanning direction. The original image data, comprising 12×10 pixels, is divided into three areas. A first area, defined as an area 0, is an area from a first to a fourth pixel (in the main scanning area, the same applies hereinafter.) A second area, defined as an area 1, is an area from a fifth to an eighth pixel. A third area, defined as area 2, is an area from a ninth to a twelfth pixel. Further, a magnification ratio in the sub-scanning direction is +25[%] (enlarged by 25[%]) in the area 0, 0[%] in the area 1, and −25[%] (reduced by 25[%])) in the area 2. That is, in the area 0, one line is inserted for every four lines and the size in the sub-scanning direction is enlarged by 25[%]. In the area 2, one line is removed for every four lines and the size in the sub-scanning direction is reduced by 25[%]. FIG. 19 shows the image data obtained after magnifying the image data in this manner.
For example, when scanning the pixel line of line d in FIG. 19, it is necessary to refer the image data of four lines in total, from a seventh line to a tenth line of the original image data. Therefore, a line buffer capable of storing the read image data of four lines in total is required. Otherwise, the line d cannot be scanned. Thus, when a shortage of the line buffer occurs, unexpected image data may be output, which causes the defective image in printing.
It is a main object of the present disclosure to provide an image forming apparatus which inhibits occurrence of defective image caused when buffer consumption amount in a sector correction exceeds a previously estimated capacity. Further, an image forming apparatus which is capable of optionally selecting, by a user, the correction processing method is provided.